When discussing carburetor "leaks", it is important to determine the meaning of the word "leak" to the original poster.
"Leak" may be anything from fuel seeping or weeping through a gasket and causing a stain over a long period of time to fuel spurting out of some hole (normally the vent) with each action of the fuel pump.
It is quite common to blame any fuel related problem on a Rochester B, if that is what is being used, and often the B is at fault; but not always. And even though I do not like the B, I don't wish to see it falsely maligned.
So:
The gasoline stain in the picture posted by the OP is probably simply a seep, which is more of a political problem than a carburetor problem. The volatility of modern fuel increases its ability to "wick". Some original carburetor gaskets (and some modern gaskets found in cheaper kits) were/are vegetable fiber. The vegetable fiber is porous, and if the retaining screws back off slightly, or even if they do not, some fuel can wick though (or around) the gasket, creating stains such as pictured. Normally, such a "leak" is so minor, the stain will never even appear to be wet; and is more of an aggravation to the enthusiast than a real problem. Warping of the air horn (common with the B) can create this situation. Filing or milling the gasket surface flat by a well-meaning but mis-informed enthusiast will make the situation worse, as the sealing ridges placed on the gasket surfaces by the manufacturer will be removed.
The most common "leak" that I see today is fuel running out beside the throttle shaft immediately after the engine is turned off. This also is a political problem, not a carburetor problem. The cause again is fuel volatility. When the engine is turned off, the inlet check valve is the fuel pump closes. This prevents fuel draining back to the fuel tank. The residual heat in the engine now heats the fuel line between the fuel pump and the carburetor. Because of fuel volatility, pressure is created in the line to a value which the combination of carburetor fuel valve and carburetor float can not withstand. A certain amount of fuel is forced past the fuel valve, raising the fuel level in the bowl. The volume is dependent on a number of variables ie ambient, length of fuel line, diameter of fuel line, and the distance of the fuel line from the engine. The additional fuel in the carburetor bowl can raise the fuel level in the bowl higher than the main discharge nozzle(s); thus fuel will drain from the bowl through the main discharge nozzle(s). As the main discharge nozzle(s) exit above the throttle plate(s), fuel will drip down to the throttle plate. What happens next is dependent upon how the owner/mechanic set the idle. If a vacuum gauge were used to adjust for the highest vacuum reading (always occurs when the throttle valve is completely closed), fuel will then run out beside the throttle shaft and drip on the manifold. If the idle was properly set, and a slight amount of clearance was available between the throttle plate and the throttle bore; then the fuel will drop down into the intake manifold, where it may create difficult starting.
The spurting leaks normally will be caused by a defective float(which would include a stuck float), defective fuel valve, or a defective fuel pump (too much pressure).
While it may be possible for a radically incorrect float setting to cause such a leak, this is unlikely. Generally, an incorrect float setting (assuming that the rebuilder is using the correct float) will cause either a mixture that is too lean or too rich.
There are other possibilities such as a blown diaphragm on a Holley type AA-1; but the majority are discussed above.
Please note the political problem (volatility) is a problem for ALL carburetors. The folks with the performance large V-8's and 1/2 inch or 5/8 inch fuel line see a lot more of these problems than folks with sixes.
The problem of fuel running by the throttle shaft may be minimized or even eliminated by installing a three line filter at the carburetor, and running a return line from the vapor port of the filter back to the fuel tank and dump in the top of the fuel tank.
However, generally the most practical way of dealing with the fuel stain pictured is a toothbrush and toothpaste
Jon.
